Just advice from a parent traveling with a teen (who is normally really responsible and careful.) With my son then 14, we made the mistake of assigning our credit card to all the keys when we checked in.
Needless to say one evening in the teen disco he could not locate his key. We found this out (after he searched for an hour everywhere) at about 1am!
We went directly to Purser's desk and they cancelled old key account and re-issued a new Sign & Sail. However we had to wait until all accounts were posted for the evening at 5am to see if any charged were made to the lost key.
Thankfully not. If a child needs to charge--have them get cash, or the fountain card. I cannot see a reason for a charge account attached. Just being cautious after our mishap.

I've always given my teen charge power on the sail and sign and never had anything unfortunate happen. He's pretty good at keeping up with his money, a wallet and his card. Maybe one of those lanyard things around the neck might work.

I have never activated my kids (teens) cards. I buy them a soda card and a t-shirt from the gift shop. There's really nothing else they need (they bring their own $ for the arcade---but don't really spend any time in there anyway). They are very responsible, but it's just one less thing to worry about (losing/misplacing the card).....

Might not be anything else my teen needs, but he found some things he wanted!

It's funny, but one of the things he really enjoys when we cruise is the power of that card. We discuss ahead of time what appropriate spending amounts are and then the decisions on how he reaches those amounts are up to him. He's bought a disposable camera, M&M's, a non-alcoholic drink, lattes, a book, a few trinkets for friends - just the stuff you find in the gift shop.

I figure it's good practice for living on a budget. He has never gone crazy with the card and I've enjoyed watching him be responsible - something you can't always say about a teen!